The immediate aim of the proposed research is to extend our knowledge and scope of systemic two-stage carcinogenesis. Its ultimate purpose is to enable one to apply standard analytical procedures to the separate stages of initiation and promotion in systemic carcinogenesis, as has been so successfully applied in the past to skin carcinogenesis. The proposed experiments may be considered under 2 headings: (1) the search for new initiators (e.g., tritiated thymidine) and potential promoters (e.g., various hormones) applicable to two-stage systemic carcinogenesis, and (2) the testing of new methods for two-stage systemic carcinogenesis. The latter would include: (a) transplacental carcinogenesis, with the initiator administered to the pregnant mother and the promoter to the offspring after birth, and (b) the adapation of intrasplenic transplantation of endocrine organs, to be performed in hypophysectomized animals (in order to repress carcinogenesis), with subsequent administration of different hormones, separately or in combination, to serve as potential promoters.